1. Field of the Invention
This invention relates to marine seismic streamer-cables and more particularly to cable-noise attenuation.
2. Description of the Prior Art
A seismic streamer-cable consists of a water-tight tubular plastic jacket containing a plurality of hydrophones, depth transducers, and electrical conductors extending through a series of bulkheads or spacers. The bulkheads are interconnected by steel strain members. The strain members prevent the plastic jacket from stretching while in tow. Streamer-cable bulkheads serve to protect the enclosed hydrophones and electrical conductors, prevent entanglement of the stress members, and maintain a cylindrical profile of the plastic jacket. The jacket is filled with a non-corrosive, non-conductive cable oil such as light kerosene to provide neutral bouyancy. For a description of the method and apparatus for seismic cable assembly, see U.S. Pat. No. 4,296,481 by Weiss and U.S. Pat. No. 3,885,286 by Hill.
In marine seismic exploration, movement of the streamer-cable through the water may generate noise on the order of 5 to 10 .mu..mu.V or more. Two mechanisms are believed to cause much of the noise, which propagates within the jacket in a waveguide mode. Hereinafter, noise traveling within the jacket in a waveguide mode will be referred as tube-wave noise.
In the first mechanism, tube-wave noise is initiated by some sound-generating source outside the jacket (a boat, streamer-cable binding causing turbulence, etc.). The resulting wave front initiates a second wave front inside the cable. The latter wave front reflects off the boundary between the cable oil and the surrounding water (the plastic jacket can be ignored) at an angle less than or equal to the critical angle, propagating within the streamer-cable as tube-wave noise almost without attenuation.
A second mechanism may occur when the strain members are subjected to tensional variations, causing axial translation of the bulkheads within the plastic tube. That movement generates compressional acoustic waves that propagate through the cable oil. Both mechanisms occur when the velocity of sound in the cable oil is less than the velocity of sound in the external medium, that is, the water.